President Obama fired his opening salvo Monday in a second-term clash with Republicans over climate change, nominating for the Environmental Protection Agency and Energy Department a duo likely to pursue further executive actions to cool global temperatures.

In tapping EPA veteran Gina McCarthy to head the environmental agency and Massachusetts Institute of Technology physicist Ernest Moniz for the Energy Department, Obama sent a clear signal to conservatives that, as he promised, global warming would receive greater attention in coming months.

From the East Room of the White House Monday, Obama predicted his nominees, who need Senate confirmation, would ensure "that we're doing everything that we can to combat the threat of climate change."

The EPA has long been a pinata for Republicans who contend that administration regulations have stifled business growth in the name of curtailing carbon emissions. McCarthy, now head of EPA's air pollution division, is an architect of Obama's efforts to limit greenhouse gases from newly constructed power plants and curtail mercury pollution from burning coal.

Advocates now want Obama to extend the tighter rules for future power plans to existing facilities -- a position McCarthy would likely spearhead if climate-change legislation goes nowhere on Capitol Hill.

"Her marching orders are very clear," Frank O'Donnell, president of the nonpartisan Clean Air Watch, said of McCarthy. "President Obama doesn't want a caretaker at the EPA. The smoking elephant in the room is the power plant. She is there to get things done."

McCarthy has served in both Republican and Democratic administrations -- including then-Massachusetts Gov. Mitt Romney's -- and Obama is banking that her bipartisan resume will ease a contentious relationship between the White House and conservative lawmakers over environmental policy.

However, some Republicans immediately pounced on McCarthy's nomination, accusing her of stonewalling lawmakers' requests for information on clean air regulations.

"You and other high-ranking administration officials have repeatedly backtracked and reneged on promises ... to make the scientific information that underpins the agency's basic associations between air quality and mortality available to the public and independent scientists," Sen. David Vitter, R-La., and Rep. Lamar Smith, R-Texas, said in a letter to McCarthy Monday.

For some detractors, however, the McCarthy nomination will serve as more of a proxy fight over Obama's policies on climate change.

"Given that other nations are not likely to follow our lead [on tighter rules for existing power plants]," said Scott Segal, a lawyer with Bracewell & Giuliani, a Houston law firm that represents energy companies, "such an outcome would result in jobs lost, projects scuttled, energy prices increased -- all with no change in global warming."

Both McCarthy and Moniz must be confirmed by the Senate, where Republicans who have challenged Obama's other second-term Cabinet picks are likely to focus on controversial issues like the Keystone XL Pipeline, cap-and-trade legislation and the bankruptcy of taxpayer-funded green energy firms -- issues they contend prove Obama's big-government solutions failed.

But with little appetite on Capitol Hill for climate change legislation, Obama is likely to turn to his new nominees for major environmental policy efforts.

"The name McCarthy will become synonymous with whatever the president pursues on climate change heading forward," said one senior Democratic Senate aide. "She'll be an easy target -- get ready for an avalanche of, um, stuff coming from the other side."